1. Field of the Invention
This invention relates in general to tunable resonators. In particular, the invention relates to the use of a novel high frequency resonant structure which in the embodiment illustrated employs microelectromechanical techniques to achieve a high quality factor and precision tuning, for use in applications such as filters and voltage-controlled oscillators.
2. Background Art
Filters are crucial components of reliable radio-frequency (xe2x80x9cRFxe2x80x9d) and microwave systems. For wireless systems to become increasingly compact and miniaturized, similarly compact filters are necessary. Furthermore, versatile systems typically require filtration of RF signals spanning widely varying frequency ranges. Thus, it is highly desirable to develop a compact filter that can be rapidly and reliably tuned over a wide frequency range.
Prior art tunable filters currently employ various types of tunable resonant structures to determine the filter""s frequency response. One prior art tunable resonator is a switched-short tunable stub. The resonant frequency of a structure such as a microstrip half or quarter-wavelength resonator is determined in part by its physical length. Because the actual physical length of a microstrip is difficult to vary dynamically, prior art switched-short techniques have controlled a resonator""s electrical length by placing a series of short circuits that can be switched open or closed spaced along the length of the resonant structure. In operation, a switch can be closed at a chosen position along the microstrip resonator to introduce a short circuit at that location and effectively set the electrical length of the resonator.
However, the foregoing switched-short structure suffers numerous potential drawbacks. Firstly, RF switches used in such structures are typically comprised of PIN diodes. However, PIN diodes suffer substantial power consumption due to forward biasing, high cost, and non-linearity. Another option that has been proposed for use as an RF switch in resonant structures utilizes microelectromechanical systems (xe2x80x9cMEMSxe2x80x9d) technology. A MEMS switch comprises a metallic bridge that can be temporarily collapsed into a conductive position via electrostatic attraction. Upon removal of the electrostatic force, the collapsed bridge of rigid metal reverts to its original shape, thereby xe2x80x9copeningxe2x80x9d the switch. However, switched-short resonant structures utilizing MEMS switches require one switch for each possible tuning position; thus, a large number of MEMS switches must be fabricated for highly tunable structures. This large number of switches results in increased manufacturing costs, and reduced reliability. It is therefore an object of this invention to provide a MEMS tunable resonator which enables a large number of tuning combinations while only requiring the fabrication of a small number of MEMS switches.
The prior art switched-short structures also suffer a low quality factor. While a MEMS switch would ideally provide an absolute short circuit at its selected position on the resonator, in reality a finite amount of electrical resistance is necessarily introduced by the metallic switch structure. Furthermore, on the switched-short resonant structure the resistance of the MEMS switch is inherently located at a current maximum on the resonator standing wave, thereby maximizing the undesired power dissipation in the switch. This non-ideality substantially limits the quality factor that can be attained by prior art resonators employing the MEMS switched-short structure. In turn, filters fabricated with such low quality factor resonators have insufficient frequency selectivity for many applications. Therefore, it is a further object of this invention to provide a MEMS tunable resonant structure that can achieve an extremely high quality factor.
Another prior art method of tuning resonant structures is by applying a varactor at the end of the structure. Typically, prior art varactor-loaded resonators have utilized a solid state varactor diode placed at the end of a quarter-wave or half-wave structure. The diode is then tuned using an analog control signal. However, because the solid state varactor requires an analog bias to control tuning, it is highly susceptible to line noise and phase noise that may be coupled onto the bias line from surrounding circuitry. It is therefore an object of this invention to provide a resonator that is tuned digitally, thereby avoiding the susceptibility to noise that is introduced by an analog control signal.
When a filter is created using varactor-loaded resonators, the filter transfer function is inherently nonlinear because prior art varactors typically exhibit nonlinear characteristics. As a result of such a nonlinear filter transfer function, filters formed with varactor-loaded resonators typically suffer very low second order and third order intercept points. Thus, varactor-loaded resonators are often only useful for a limited number of applications, such as receivers exposed only to extremely low power levels. It is therefore an object of this invention to provide a versatile tunable filter with a highly linear transfer function.
Prior art filters using varactor-loaded resonators also suffer high insertion loss due to the significant series resistance inherent in varactor diodes. The insertion loss problem becomes particularly significant when multiple resonators are required to achieve a desired filter performance. Therefore, it is an object of this invention to minimize the insertion loss inherent in the use of a tunable resonant structure.
While varactors fabricated using MEMS techniques have been proposed to replace the solid-state varactors previously utilized in varactor-loaded resonant structures, both MEMS and solid-state varactors are significantly limited in their usable capacitance variation. Prior art MEMS varactors are typically limited to a capacitance variation of approximately 1.3:1. Therefore, neither MEMS nor solid-state varactor-loaded resonators offer a wide tuning range. It is therefore an object of this invention to provide a tunable resonant structure employing MEMS technology to implement a very wide tuning range.
Some prior art filter designs utilize multiple resonators that are capacitively coupled together. However, the coupling coefficients of typical prior art capacitive coupling techniques vary over frequency. When a tunable filter employs such coupling, the varying coupling coefficients may alter the filter response as it is tuned across a broad frequency range. Because such variation is undesirable in many applications, it is an object of this invention to provide a structure with a variable, tunable coupling coefficient.
These and other objects of the present invention will become apparent to those of ordinary skill in the art in light of the present specifications, drawings and claims.
The invention allows for the tuning of a radio frequency or microwave resonator over a wide frequency bandwidth, thereby providing for the implementation of high quality-factor tunable filters. The tunable resonator is comprised of a microstrip configuration of predetermined length.
Microelectromechanical switches are located at one or more positions along the length of the microstrip. The switches are MEMS bridges comprised of spans of a metal membrane crossing over the microstrip, with an air gap between the membrane and microstrip. Each bridge is also connected at one end to a radial stub, which can act as a capacitive load. When an electrostatic potential differential is applied between the bridge and the microstrip, the bridge collapses, thereby forming an electrical connection between the microstrip and radial stub. The radial stub loads the microstrip to create a slow wave structure, thereby lowering the resonant frequency of the microstrip. When the electrostatic potential differential between the bridge and microstrip is removed, the bridge reverts to its prior position above the microstrip, thereby disconnecting the load from the microstrip, and increasing the resonant frequency of the resonator. A large number of resonator tuning states can be achieved as multiple switches at various positions along the resonator engage and disengage the various capacitive loads.
Multiple resonator stubs can be combined to create various filter configurations, as is known in the art. Resonator stubs can be coupled using direct connections or capacitive air gaps. However, because filters created using the disclosed tunable resonators can cover a wide tuning frequency range, it may also be desirable. to control the coupling coefficient to resonators by implementing a tunable coupling configuration. One or more MEMS bridges span a first microstrip. Each MEMS bridge is separated from a resonator microstrip by a predetermined capacitive air gap. When a bridge is collapsed into a closed state by an electrostatic potential differential between it and the first microstrip which it spans, the bridge becomes coupled with the first microstrip, such that the first microstrip is further coupled to the resonator microstrip via the predetermined capacitive air gap between the resonator and the bridge. When the electrostatic potential differential is eliminated, the bridge returns to its open state and the microstrips are no longer coupled by the predetermined capacitive air gap associated with the bridge. The first microstrip and the resonator microstrip can also be positioned in close proximity such that they are capacitively coupled via a permanent air gap even when each coupling bridge is in an open state. Thus, the coupling capacitance between microstrips can be adjustably controlled.